fix #19, advanceBy firstly

__tests__/index.js

@@ -6,14 +6,12 @@
 import { advanceBy, advanceTo, clear, version } from '../src';
 import pkg from '../package.json';
 
-beforeEach(() => {
-  advanceTo(); // advanceTo to 0 ms.
-});
 afterEach(() => {
   clear();
 });
 describe('jest-date-mock', () => {
   test('Date Class', () => {
+    advanceTo();
     // Date isEqual
     expect(new Date()).toEqual(new Date(0));
 
@@ -68,14 +66,15 @@ describe('jest-date-mock', () => {
   });
 
   test('advanceBy', () => {
-    advanceTo(1000);
     advanceBy(3000); // advanceBy time 3 seconds
-    expect(+new Date()).toBe(4000);
+    const now = new Date();
+    advanceBy(4000);
+    expect(+new Date() - now).toBe(4000);
 
     advanceBy(-4000); // advanceBy time -4 seconds
-    expect(+new Date()).toBe(0);
+    expect(+new Date() - now).toBe(0);
     advanceBy();
-    expect(+new Date()).toBe(0);
+    expect(+new Date() - now).toBe(0);
   });
 
   test('clear', () => {

package.json

@@ -1,11 +1,11 @@
 {
   "name": "jest-date-mock",
-  "version": "1.0.6",
+  "version": "1.0.7",
   "description": "Mock `window.Date` when run unit test cases with jest. Make tests of `Date` easier.",
   "main": "lib/index.js",
   "typings": "lib/index.d.ts",
   "scripts": {
-    "test": "jest",
+    "test": "jest --no-cache",
     "build": "babel src --out-dir lib --copy-files && npm run test",
     "coveralls": "cat ./coverage/lcov.info | coveralls"
   },

src/date.js

@@ -3,6 +3,7 @@
  * Contract: [email protected]
  */
 
+// undefined means unmock.
 const DEFAULT = undefined;
 
 // current Date of timestamp
@@ -12,7 +13,7 @@ let nowDate = DEFAULT;
  * move date by offset `ms`
  * @param ms
  */
-export const advanceBy = ms => nowDate += (ms || 0);
+export const advanceBy = ms => nowDate = (nowDate === undefined ? +new Date() : nowDate) + (ms || 0);
 
 /**
  * reset Date

src/index.d.ts

@@ -4,15 +4,15 @@
  * @param ms
  * @default 0
  */
-export declare function advanceBy(ms: number): void;
+export declare function advanceBy(ms?: number): void;
 
 /**
  * Sets date to a timestamp or Date.
  *
  * @param ms
  * @default 0
  */
-export declare function advanceTo(ms?: number | Date): void;
+export declare function advanceTo(ms?: number | Date | string): void;
 
 /**
  * Un-mocks the Date class.

src/mockDate.js

@@ -6,6 +6,7 @@
 import { now } from './date';
 
 export const mockDateClass = D => {
+  // if undefined, use real date, or else mock date
   const mockNow = () => now() === undefined ? D.now() : now();
 
   function MD(...args) {